[TIR] Keep trivial LetStmt in tir.Simplify when used in buffer decl

Prior to this commit, any trivial let binding of `var1 = var2` is
inlined.  However, buffer definitions are not updated, so this can
result in dangling `tir::Var` instances.  This commit updates the
`tir.Simplify` pass to keep trivial let bindings if they are used as
part of a buffer definition.

Ideally, the trivial `LetStmt` variable would be inlined into the
buffer definition as well as other expressions.  However, because a
buffer may be implicitly declared, the first usage may be within a
constrained context.  If that happens, the simplified shape/strides
expression cannot be used to update the buffer definition, as that
simplification is not valid at all possible usage points of the
buffer.

```python
for i in range(n):
    elem_offset = i
    view = T.Buffer(1, data=buf, elem_offset = elem_offset)
    if i == 0:
        # First occurrence in TIR is here, where elem_offset would
        # simplify to zero.
        view[0] = 1
    else:
        # But the same buffer is used here, where elem_offset doesn't
        # simplify to zero.
        view[0] = 2
```

This will be resolvable after apache#14778
lands, requiring all buffers to be declared with `DeclBuffer` prior to
usage.

```python
for i in range(n):
    elem_offset = i
    # All variables used by the DeclBuffer are valid across the entire
    # body of the DeclBuffer.
    view = T.decl_buffer(1, data=buf, elem_offset = elem_offset)
    if i == 0:
        view[0] = 1
    else:
        view[0] = 2
```

src/tir/transforms/simplify.cc

@@ -33,6 +33,7 @@
 
 #include "../../arith/ir_mutator_with_analyzer.h"
 #include "../../tir/analysis/control_flow_graph.h"
+#include "../../tir/analysis/var_use_def_analysis.h"
 
 namespace tvm {
 namespace arith {
@@ -91,6 +92,46 @@ struct SimplifyConfigNode : public tvm::AttrsNode<SimplifyConfigNode> {
   }
 };
 
+/* \brief Utility function to collect vars that should be retained */
+std::unordered_set<const VarNode*> CollectVarsUsedInBufferDefinition(const Stmt& stmt) {
+  struct Visitor : StmtExprVisitor {
+    using StmtExprVisitor::VisitExpr_;
+    using StmtExprVisitor::VisitStmt_;
+
+    void VisitExpr_(const BufferLoadNode* op) override {
+      VisitBuffer(op->buffer);
+      StmtExprVisitor::VisitExpr_(op);
+    }
+    void VisitStmt_(const BufferStoreNode* op) override {
+      VisitBuffer(op->buffer);
+      StmtExprVisitor::VisitStmt_(op);
+    }
+
+    void VisitBuffer(const Buffer& buf) {
+      // Collect variables that should remain defined
+      VarUseDefAnalyzer usage(Array<Var>{});
+      usage(buf->data);
+      for (const auto& dim : buf->shape) {
+        usage(dim);
+      }
+      for (const auto& dim : buf->strides) {
+        usage(dim);
+      }
+      usage(buf->elem_offset);
+
+      // Track for use in LetStmtNode mutator
+      for (const auto& var : usage.undefined_) {
+        used_in_buffer_def_.insert(var.get());
+      }
+    }
+    std::unordered_set<const VarNode*> used_in_buffer_def_;
+  };
+
+  Visitor visitor;
+  visitor(stmt);
+  return visitor.used_in_buffer_def_;
+}
+
 class SimplifyConfig : public Attrs {
  public:
   TVM_DEFINE_NOTNULLABLE_OBJECT_REF_METHODS(SimplifyConfig, Attrs, SimplifyConfigNode);
@@ -110,16 +151,24 @@ class StmtSimplifier : public IRMutatorWithAnalyzer {
         config->propagate_knowns_to_simplify_expressions) {
       touch_pattern = ControlFlowGraph(stmt);
     }
-    StmtSimplifier simplifier(analyzer, config, std::move(touch_pattern));
+
+    std::unordered_set<const VarNode*> used_in_buffer_def = CollectVarsUsedInBufferDefinition(stmt);
+    StmtSimplifier simplifier(analyzer, config, std::move(touch_pattern),
+                              std::move(used_in_buffer_def));
     return simplifier(std::move(stmt));
   }
 
  private:
   explicit StmtSimplifier(Analyzer* analyzer, SimplifyConfig config,
-                          std::optional<ControlFlowGraph> touch_pattern)
-      : IRMutatorWithAnalyzer(analyzer), config_(config), touch_pattern_(touch_pattern) {}
+                          std::optional<ControlFlowGraph> touch_pattern,
+                          std::unordered_set<const VarNode*> used_in_buffer_def)
+      : IRMutatorWithAnalyzer(analyzer),
+        config_(config),
+        touch_pattern_(touch_pattern),
+        used_in_buffer_def_(used_in_buffer_def) {}
 
   using Parent = IRMutatorWithAnalyzer;
+  using Parent::VisitExpr_;
   using Parent::VisitStmt;
   using Parent::VisitStmt_;
 
@@ -159,18 +208,36 @@ class StmtSimplifier : public IRMutatorWithAnalyzer {
 
   Stmt VisitStmt_(const LetStmtNode* op) override {
     PrimExpr value = this->VisitExpr(op->value);
-    if (CanInlineLetStmt(op)) {
-      // it is fine to discard the let binding
-      // because the call to simplify will always inline the var.
+    bool can_inline = CanInlineLetStmt(op);
+    if (can_inline) {
+      // It is usually fine to discard the let binding because the
+      // call to simplify will always inline the var.
+      //
+      // The exception is when the variable is used in a Buffer's
+      // definition, as these are not updated by the simplification.
+      // After DeclBuffer is required prior to use of a buffer,
+      // simplifying can update the buffer definition as well.  The
+      // buffer can only be updated at its point of definition,
+      // because the points of use may occur within contexts that
+      // allow for additional simplifications (e.g. a buffer of shape
+      // [i,j] whose first use occurs within "if i==1" should not have
+      // its shape simplified to [1,j]).
       analyzer_->Bind(op->var, value);
-      return this->VisitStmt(op->body);
     } else if (SideEffect(op->value) <= CallEffectKind::kPure) {
       // Even if we aren't replacing all occurrences, they may be
       // necessary for proving conditional statements.
       non_inlined_bindings_.Set(op->var, value);
     }
     Stmt body = this->VisitStmt(op->body);
-    if (value.same_as(op->value) && body.same_as(op->body)) {
+
+    // TODO(Lunderberg): Update the Buffer object as part of
+    // DeclBuffer updates, which will first require
+    // https://github.com/apache/tvm/pull/14778.
+    bool used_in_buffer_def = used_in_buffer_def_.count(op->var.get());
+
+    if (can_inline && !used_in_buffer_def) {
+      return body;
+    } else if (value.same_as(op->value) && body.same_as(op->body)) {
       return GetRef<Stmt>(op);
     } else {
       auto n = this->CopyOnWrite(op);
@@ -207,8 +274,10 @@ class StmtSimplifier : public IRMutatorWithAnalyzer {
     return Parent::VisitExpr_(op);
   }
 
+  PrimExpr VisitExpr_(const BufferLoadNode* op) override { return Parent::VisitExpr_(op); }
+
   // eliminate useless stores
-  Stmt VisitStmt_(const BufferStoreNode* op) final {
+  Stmt VisitStmt_(const BufferStoreNode* op) override {
     BufferStore store = Downcast<BufferStore>(Parent::VisitStmt_(op));
     if (const BufferLoadNode* load = store->value.as<BufferLoadNode>()) {
       if (load->buffer->data.same_as(store->buffer->data) &&
@@ -260,6 +329,7 @@ class StmtSimplifier : public IRMutatorWithAnalyzer {
 
   Map<Var, PrimExpr> non_inlined_bindings_;
   Optional<Stmt> current_stmt_{NullOpt};
+  std::unordered_set<const VarNode*> used_in_buffer_def_;
 };
 
 }  // namespace arith

tests/python/unittest/test_tir_transform_simplify.py

@@ -1689,5 +1689,49 @@ def expected(A: T.Buffer(1, "int32")):
         A[0] = 12
 
 
+class TestSimplifyTrivialLetBufferVar(BaseBeforeAfter):
+    """A LetStmt used in a buffer definition should be retained"""
+
+    def before(A_ptr: T.handle("float32")):
+        A_ptr_redef: T.handle("float32") = A_ptr
+        A = T.decl_buffer(1, "float32", data=A_ptr_redef)
+        A[0] = 42.0
+
+    expected = before
+
+
+class TestSimplifyTrivialLetElemOffset(BaseBeforeAfter):
+    """A LetStmt used in a buffer definition should be retained"""
+
+    def before(A_ptr: T.handle("float32"), A_offset: T.int32):
+        A_offset_redef = A_offset
+        A = T.decl_buffer(1, "float32", elem_offset=A_offset_redef, data=A_ptr)
+        A[0] = 42.0
+
+    expected = before
+
+
+class TestSimplifyTrivialLetShape(BaseBeforeAfter):
+    """A LetStmt used in a buffer definition should be retained"""
+
+    def before(A_ptr: T.handle("float32"), A_size: T.int32):
+        A_size_redef = A_size
+        A = T.decl_buffer([A_size_redef], "float32", data=A_ptr)
+        A[0] = 42.0
+
+    expected = before
+
+
+class TestSimplifyTrivialLetStride(BaseBeforeAfter):
+    """A LetStmt used in a buffer definition should be retained"""
+
+    def before(A_ptr: T.handle("float32"), A_stride: T.int32):
+        A_stride_redef = A_stride
+        A = T.decl_buffer(1, "float32", strides=[A_stride_redef], data=A_ptr)
+        A[0] = 42.0
+
+    expected = before
+
+
 if __name__ == "__main__":
     tvm.testing.main()